צריך לסמוך גאולה לתפלה ולא יפסיק ביניהם אפי' באמן אחר גאל ישראל ולא בשום פסוק חוץ מה' שפתי תפתח: הגה וי"א שמותר לענות אמן על גאל ישראל וכן נוהגין (טור) וי"א הא דצריך לסמוך גאולה לתפלה היינו דוקא בחול או ביו"ט אבל בשבת א"צ (פי' דטעמא דבעינן למסמך גאולה לתפלה משום דכתיב יענך ה' ביום צרה וסמיך ליה יהיו לרצון אמרי פי ושבת לאו זמן צרה ולענ"ד נראה דמה שאין כן ביו"ט הוא משום שהם ימי הדין כדתנן במשנה ב' פ"ק דר"ה בפסח על התבואה וכו') (הגהות אשיר"י פ"ק דברכות וכל בו הלכות שבת ומהרי"ל הל' י"ט) וטוב להחמיר אם לא במקום שצריך לכך (טור):

One needs to juxtapose “redemption” [i.e. the last blessing of the Sh'ma - "Ga-al Yisrael"] to “prayer” [i.e. the Amidah]. And one should not interrupt between them, even with "Amen" after “Ga-al Yisrael”, and not for any verse other than “Hashem Sefatai” [Psalms 51:17, the introductory verse for the Amidah]. Gloss: And there are those who say that it is permitted to respond Amen after “Ga-al Yisrael”, and so we practice (Tur). And there are those who say that this requirement to juxtapose “redemption” to “prayer” is only on a weekday or Yom Tov, but on Shabbat one does not need to. (Meaning, that the reason that we require to juxtapose “redemption” to “prayer” [during the week] is because it is written “God will answer you in a day of distress,” [Psalms 20:2] and juxtaposed to it [is written], “Let the words of my mouth be desire … and my redeemer.” [Psalms 19:15], but Shabbat is not a time of distress. And in my humble opinion, it seems [that the reason] this is not so on Yom Tov is because they are days of judgement, as we learned in Mishnah Rosh Hashanah 1:2, “On Passover [the world is judged] on grain…”) (Hagahot Ashir"i [Rabbi Yisrael MiKrems] on Chapter 1 of Berachot; and Kol Bo, Laws of Shabbat; and Mahari"l, the Laws of Yom Tov) And it is good to be stringent unless in a place that needs to do such (Tur).

The prayer leader, when beginning the recitation of the Eighteen [Blessings i.e. Amidah] out loud, goes back [to the beginning] and says: "Lord, open my lips and my mouth shall declare, etc."

If one found the congregation praying [the Amidah], when one has not yet recited the Recitation of the Shema, one should not pray with them, rather one should recite the Recitation of the Shema and subsequently pray, since juxtaposing “redemption” to “prayer” is preferred.